How to count the no. of times the given character in second TextBox appears in First Textbox and add the index and matched character to ListBox

MainWindow

 <Window x:Class="WPFWorldTutorial.Views.MainWindow"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        xmlns:prism="http://prismlibrary.com/"

        prism:ViewModelLocator.AutoWireViewModel="True">

    <Grid>

        <StackPanel Orientation="Vertical">

            <TextBox Text="{Binding Text}" Height="25" Margin="5"/>

            <TextBox Text="{Binding CharToMatch}" Height="25" Margin="5"/>

            <Button Content="Match" Command="{Binding MatchCharCommand}" Width="100" Margin="5"/>

            <ListBox ItemsSource="{Binding MatchedInfo}">

                <ListBox.Style>

                    <Style TargetType="ListBox">

                        <Setter Property="Visibility" Value="Visible"/>

                        <Style.Triggers>

                            <DataTrigger Binding="{Binding MatchedInfo.Count}" Value="0">

                                <Setter Property="Visibility" Value="Collapsed"/>

                            </DataTrigger>

                        </Style.Triggers>

                    </Style>

                </ListBox.Style>

                <ListBox.ItemTemplate>

                    <DataTemplate>

                        <StackPanel Orientation="Horizontal">

                            <TextBlock Text="{Binding Charecter}" Margin="5"/>

                            <TextBlock Text="->" Margin="5"/>

                            <TextBlock Text="{Binding Position}" Margin="5"/>

                        </StackPanel>

                    </DataTemplate>

                </ListBox.ItemTemplate>

                </ListBox>

        </StackPanel>

       

    </Grid>

</Window>

CharInfo.cs

Here BindableBase class i used from the prism if you are not using Prism you will not get BindableBase class you have to implement INotifyPropertyChanged

 public class CharInfo : BindableBase

    {

        private string _charecter;

        public string Charecter

        {

            get { return _charecter; }

            set

            {

                _charecter = value;

                SetProperty(ref _charecter, value);

            }

        }

        private int _position;

        public int Position

        {

            get { return _position; }

            set

            {

                _position = value;

                SetProperty(ref _position, value);

            }

        }

    }

Add these Property, Command and method to ViewModel

 private ObservableCollection<CharInfo> matchedInfo = new ObservableCollection<CharInfo>();

        public ObservableCollection<CharInfo> MatchedInfo { get => matchedInfo; set => SetProperty(ref matchedInfo, value); }

        private string text;

        public string Text { get => text; set => SetProperty(ref text, value); }

        private string charToMatch;

        public string CharToMatch { get => charToMatch; set => SetProperty(ref charToMatch, value); }

        private ICommand matchCharCommand;

        public ICommand MatchCharCommand

        {

            get

            {

                if (matchCharCommand == null)

                {

                    matchCharCommand = new DelegateCommand(MatchChar);

                }

                return matchCharCommand;

            }

        }

        private void MatchChar()

        {

            MatchedInfo.Clear();

            if (!string.IsNullOrEmpty(Text) && !string.IsNullOrEmpty(CharToMatch))

            {

                for (int i = 0, pos = -1; i < Text.Length;)

                {

                    pos = Text.IndexOf(CharToMatch, pos + 1);

                    if (pos < 0)

                        break;

                    MatchedInfo.Add(new CharInfo { Charecter = charToMatch, Position = pos });

                    i = pos + 1;

                } 

            }

        }